System and method for use in functional failure analysis by induced stimulus

ABSTRACT

A scanning/imaging system wherein an external stimulus is used for exciting a device under test (DUT). A stimulus source is included for providing a stationary stimulus with a controllable spot size to a device under test (DUT), the controllable spot size covering a portion of the DUT for excitation by the stationary stimulus. A sensor is operable for capturing at least one of a functional response signal and an optical image signal emanating from the DUT portion. A linear positioning device is operable to facilitate scanning of remaining portions of the DUT until a predetermined area thereof has been traversed. A controller is operably coupled to the linear positioning device, stimulus source and the sensor for providing the overall control thereof.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of the following patent application:“SYSTEM AND METHOD FOR USE IN FUNCTIONAL FAILURE ANALYSIS BY INDUCEDSTIMULUS,” filed Oct. 18, 2004, application Ser. No. 10/967,808, in thename(s) of: James Barry Colvin, now U.S. Pat. No. 7,323,888 which ishereby incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention generally relates to scanning and imagingtechniques in failure analysis of electronic devices such assemiconductor integrated circuits, packages, boards, and the like. Moreparticularly, and not by way of any limitation, the present invention isdirected to a system and method for isolating failures in electronicdevices using stimulus-induced fault testing (SIFT).

2. Description of Related Art

In the field of failure analysis of integrated circuits, isolating anddiagnosing functional failures is a cardinal requirement. Traditionalbeam-based analytical techniques use a scanning laser or electron beam(e-beam) to induce a parametric shift, which is monitored throughchanges in current or voltage driven to the device. Deep submicrontechnologies and complex, multi-layered architectures prevalent todayfrustrate these analytical methods due to the nearly immeasurableparametric shifts externally caused by a small internal leakage path.Although these failures can sometimes be identified functionally bytiming, temperature, or voltage dependencies, even the proximatelocation of the fault is difficult to isolate. Relatedly, field of viewlimitations inherent in today's beam-based techniques compound thedifficulty due to the fact that a huge number of scans need to performedto cover a large die, resulting in an enormous amount of test pointsthat require an inordinate period of time for completing the analysis.

SUMMARY OF THE INVENTION

In general, the present invention involves intentionally disturbingelectronic devices, parts, boards, etc. with an external stimulus sourceand comparing the test criteria to reference parts or timing/voltagesensitivities. In one aspect, the present invention is directed to anembodiment of a method that comprises one or more of the followingoperations: applying a stationary stimulus with a controllable spot sizeto a device under test (DUT), the controllable spot size covering aportion of the DUT for excitation by the stationary stimulus; capturingat least one of a functional response signal and an optical image signalemanating from the portion of the DUT; and scanning remaining portionsof the DUT until a predetermined area thereof has been traversed.

In another aspect, the present invention is directed to an embodiment ofa fault isolation method. The claimed embodiment comprises one or moreof the following: providing a stationary stimulus with a controllablespot size to a device under test (DUT), the controllable spot sizecovering a portion of the DUT for excitation by the stationary stimulus;capturing at least one of a functional response signal and an opticalimage signal emanating from the portion of the DUT; facilitatingscanning of remaining portions of the DUT until a predetermined areathereof has been traversed; and isolating a fault in the DUT inaccordance with a select failure analysis technique.

In a still further embodiment, the present invention is directed to asystem, comprising: means for applying a stationary stimulus with acontrollable spot size to a device under test (DUT), the controllablespot size covering a portion of the DUT for excitation by the stationarystimulus; means for capturing at least one of a functional responsesignal and an optical image signal emanating from the portion of theDUT; and means for scanning remaining portions of the DUT until apredetermined area thereof has been traversed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated into and form a part of thespecification to illustrate one or more presently preferred exemplaryembodiments of the present invention. Various advantages and features ofthe invention will be understood from the following Detailed Descriptiontaken in connection with the appended claims and with reference to theattached drawing figures in which:

FIG. 1 depicts a block diagram of an exemplary scanning systemembodiment operable for fault isolation in accordance with the teachingsof the present invention;

FIG. 2 is flow chart of an exemplary scanning method operable for faultisolation in accordance with the teachings of the present invention; and

FIG. 3 depicts an exemplary biasing circuit for use with the embodimentshown in FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference tovarious examples of how the invention can best be made and used. Likereference numerals are used throughout the description and several viewsof the drawings to indicate like or corresponding parts, wherein thevarious elements are not necessarily drawn to scale. Referring now tothe drawings, and more particularly to FIG. 1, depicted therein is ablock diagram of an exemplary scanning system or apparatus 100 operablefor fault isolation and image generation in accordance with anembodiment of the present invention. As illustrated, the system 100 maybe based on either a motorized probe station or a microscope stand fortest head applications, although other configurations are possible. Oneor more external stimulus sources 106A, 106B are operated under controlof a controller unit 102 for applying a stationary stimulus with acontrollable spot size to a device under test (DUT) positioned on astage, collectively referred to by reference numeral 112. As will bedescribed in additional detail hereinbelow, the stimuli can be a varietyof electromagnetic, electrostatic, magnetic, acoustic or radio frequency(RF) signals, optical or electron beams, etc. that are capable ofexciting a portion of the DUT covered by the variable spot size. Sincethe scanning system 100 uses a stationary or fixed stimulus with anarbitrary spot size and is not dependent on traditional microscopeoptics for generating a highly focused beam (e.g., having a diameter ofone or several microns depending on the diffraction characteristics ofthe optical elements used) that is conventionally rastered across alimited field of view, the scan area of the present invention can bevariable as well as relatively quite large, ranging from a few micronsto 12 inches or more. Accordingly, the present invention allows forscanning of a diverse array of electronic devices, such as individualdies, packages, wafers, whole boards, etc., for fault isolation and/orimaging purposes. Where the scanning system 100 is deployed in a faultisolation application, the scanning system 100 may be termed as thestimulus-induced fault testing (SIFT) scanner for purposes of thepresent patent application.

In one embodiment, stimulus source 106A is operable to provide asuitable stimulus that is directly applied within the proximity of thestage/DUT 112 for excitation of the DUT structures, e.g., as a probe tipthat fixedly hovers over the surface of the DUT. In another embodiment,stimulus source 106B is coupled to a microscope head 108 whereby thestimulus is directed via an objective piece 109 towards the stage/DUTassembly 112. In either implementation, a biasing circuit 114 may usedfor properly biasing the DUT depending upon the applied stimuli and typeof DUT as well as the particular failure analysis technique in which thescanning system is put to use.

Those skilled in the art should appreciate that various DUT structuressuch as p/n and n/p junctions, p-wells and n-wells, oxide layers,diffusion areas, contacts, inter-level vias, metal conductors andinterconnects, and nodes associated with active devices such astransistors, diodes, and so on, are operable to emanate characteristicresponse signals upon excitation by the applied stimuli, which can becaptured by appropriate sensor(s) 110. Again, depending on the appliedstimuli and the DUT type, sensors can be magnetoresistive, optical, RF,or thermal sensing elements that can provide appropriate data to thecontroller 102. A tester 104 with any type of user interface is coupledto the controller 102 for generating test vectors, patterns, conditions,etc. relative to the DUT being analyzed. The data output can bepass/fail data as well as analog data that may be converted to any n-bitdigital data or vice versa. In addition, although not shown in FIG. 1,it should recognized that other equipment such as curve tracers,parametric analyzers, charge-coupled device (CCD) cameras, liquidcrystal display (LCD) monitors, electronic components like signalanalyzers, amplifiers, and the like may also be deployed in associationwith the tester 104 of the scanning/imaging system 100.

Because the applied stimulus is a stationary stimulus, either thestage/DUT assembly 112 or the microscope head 108 is moved in the X-Yplane in a raster scan pattern so as to traverse the desired DUT area.Also, either the stage/DUT assembly 112 or the microscope head 108 iscapable of moving in the Z-direction in order to control the spot sizeof the applied stimulus by either focal control of the microscope opticsor proximity control of the stimulus source. When a laser-based scanningapplication is desired, any suitable laser (e.g., a 670 nm laser diode)that is compatible with the selected optics is attached to the topcamera port of the microscope head 108. The selected objective,aperture, as well as related focal control contribute to the regulationof the spot size in this embodiment. Alternatively, where the fixedstimulus is introduced over the DUT with the raster control beingapplied to the stage, the Z-control determines the spot size. As alludedto before, this scanning method embodiment may be thought of in the sameway a probe is positioned over the DUT by moving the stage, and may betermed the scanned probe method. It should be appreciated by thoseskilled in the art that the scanned probe method allows coupling of ascanning tip to the device being tested, whereinelectrostatic/capacitive coupling of the tip the device is useful forlocalizing floating nodes on an integrated circuit or for mapping activecircuit paths electrostatically or magnetically.

Continuing to refer to FIG. 1, controller 102 may be embodied as amicrocontroller or a processor-based system that is responsible forX-Y-Z positioning, stimulus source control, analog or digital signalcapture from the stage/DUT 112 or tester 104 where applicable, andcommunicates with the tester 104 for handshaking. Communication can beone-sided, i.e., where the controller 102 will provide a positioncontrol signal only after receiving a pass/fail signal from the tester104. On the other hand, bi-directional communication involves ready tostep and ready for next loop handshake signaling between the tester 104and the controller 102. Irrespective of the type of communication, thescan and step is preferably user-defined based on the desired spot size.

The X-Y-Z movement is effectuated by suitable motors 116 comprisingstepper motors, servo motors or similar linear positioning devicesoperating under the control of controller 102. Since the DUT is scannedin X- and Y-directions based on coordinate data, a computer-aided design(CAD) layout database of the DUT may be appropriately interfaced withthe scanning system 100 for matching scan position control with theDUT's design database itself. Accordingly, no optical imaging isabsolutely necessary in order to ascertain where on the device thescanning is being done.

FIG. 2 is flow chart of an exemplary scanning method operable for faultisolation or device imaging in accordance with an aspect of the presentinvention. A stationary stimulus is applied with a controllable spotsize to a DUT, either directly or via a microscope head (block 202). Asdescribed above, the controllable spot size is controlled in oneembodiment by moving the DUT's stage in a vertical direction (i.e.,Z-direction). In another embodiment, the spot size is regulated byvarying focal control associated with the microscope head to which astimulus source is coupled. Also, the microscope head itself may bemoved in the Z-direction to vary the spot size by defocusing. Thestationary stimulus is operable to excite the area or portion of the DUTcovered by the spot size such that response signals emanate therefrom.Using appropriate sensor arrangements, the response signals may becaptured for further processing (block 204). The response signals caninclude a photo image signal, a functional response signal, or both,that emanate from the DUT portion under stimulus. The remaining portionsof the DUT may be scanned, if necessary, by stepping the DUT's stage orthe microscope head (block 206). Accordingly, the scan and stepoperations may iteratively continue until the entire area of the DUT hasbeen traversed (e.g., for obtaining a photo image of a large die). Onthe other hand, the scan and step operations may continue until apredetermined area of the DUT has been scanned, e.g., a decoder block orinput/output block of a semiconductor memory.

Those skilled in the art should recognize that the scanning/imagingtechnique described above is not only elegant in its simplicity, butaffords a substantial degree of versatility as well. Because a diversearray of stimulus sources can be accommodated within the scope of thepresent invention, the scanning system 100 may be deployed in numerousfunctional modes. Some of the exemplary functional modes of the scanningsystem of the present invention are set forth immediately hereinbelow.

A. High Frequency Magnetic Induction: By placing a miniature inductioncoil over the DUT and scanning in accordance with the teachings herein,package level shorts can be mapped. Based on the frequency used (10 MHzto 20 MHz, for example), a spatial resolution of about 2 mm may beachieved. Magnetic induction couples to and heats directly theconductors embedded in a package, whereby parametric shifts of theleakage are monitored during the scanning process in relation to thecoordinates of the package floor plan. Also, mutual coupling of themagnetic field can be used to map conductor location.

B. Magnetic Current Mapping: By positioning a miniature magnetoresistivesensor within the scanner apparatus, any or all axes (X, Y, and Z) ofthe magnetic field across a DUT can be mapped. This is particularlyuseful for identifying current flow in the DUT. Abnormal or normal pathscan be mapped in the microamp (μA) level, allowing tracing and debug ofthe DUT. In this functional mode, resolution is impacted primarily bytip proximity and field strength.

C. RF Scan: In this mode, an RF probe tip is employed to inject orreceive RF energy over the DUT. This technique is advantageous fortracing circuit paths carrying their characteristic frequency, and maybe applied at both die level and package level. Opens can be identifiedby assuming that the capacitive coupling across an open is not dominant.Again, spatial resolution is impacted primarily by proximity of the DUTto the tip source.

D. Functional SIFT: In this mode, pass/fail data from the tester is usedin conjunction with the stepped or rastered stage/DUT in order togenerate a physical map of pass/fail data.

E. Electrostatic SIFT: An AC field is coupled to the device wherein theconductor lines act as a stimulus source.

F. Thermal SIFT: Thermal stimulus is useful for localizing ohmic shortsor intermittent connections as well as timing related failures. Thismode involves thermally exciting a localized area of the DUT whilescanning it. Both ohmic Electrostatic Discharge (ESD) or Vdd failuresare also suitable candidates for this technique, which may be based onthe following stimulus sources: (i) scanning hot probe tips, (ii)thermal lasers at appropriate wavelengths, and (iii) magnetic inductionheating elements.

Furthermore, there exist many different ways to electrically bias a DUTfor beam-based measurements, and all are compatible with the teachingsof the present invention. Thus, known failure analysis techniques suchas Light Induced Voltage Alteration (LIVA), Optical Beam Induced Current(OBIC) imaging, Thermally Induced Voltage Alteration (TIVA), ExternallyInduced Voltage Alteration (XIVA), Charge Induced Voltage Alteration(CIVA), Optical Beam Induced Resistance Change (OBIRCH), etc., forexample, may be advantageously practiced in conjunction with thescanning/imaging technique of the present invention for quicker faultisolation/localization and subsequent fine-grained beam raster scan ofthe site for further analysis.

FIG. 3 depicts an exemplary biasing circuit 300 for use with the scannerembodiment shown in FIG. 1. As illustrated, the biasing circuit 300 isoperable for supplying constant current type performance with constantvoltage control. A voltage supply 304 is coupled to a current source 306for driving a constant current across a DUT 302. A buffering stage 308is used for buffering the resultant voltage drop across DUT 302 at nodeA that is coupled to a low-pass filter 312. The DC value of the DUT biasis supplied by the low-pass filter 312 to a summing circuit 316 whichadds the negative desired set-point of the DUT (compliance voltageset-point) to the actual measured DUT bias. The output from the summer316 is multiplied by a predetermined value, e.g., −2.7, by a multiplier318, that controls the current source 306 such that Vcc is maintained onthe DUT. The signal at node A is also supplied to a high-pass activegain filter 310 for detection of an oxide noise signature (ONS) signal,which is monitored during a SIFT scan to identify minute shifts inoperation during a scan.

Based on the foregoing Detailed Description, it should be appreciatedthat the present invention advantageously provides a versatile scanningapparatus and method that could be utilized in fault isolationapplications in performing failure analysis or simply in imaginglarge-scale circuitry. By employing SIFT, a number of external stimulican be used to identify speed, fault, and parametric differences insilicon. Several benefits of the present invention are readily apparent:(i) large area scans beyond the limits of the field of view of themicroscope are feasible; (ii) scan speed is controllable both bystepper/servo motor speed and test time/loop; (iii) ability to set thespot size at a relatively larger size, thereby allowing rapid and/orfewer scans of a device; (iv) galvo scanner drift generally present inconventional laser scan microscopes is eliminated, whereby accurate stepand hold operations are possible; (v) mechanical motor control iscompatible with microstepping and servo positioning methods; (vi)enabling of conventional laser-based scan methods (e.g., OBIC) toanalyze the entire die or large areas of a device; (vii) remote opticalcoupling to tester or test equipment display monitors; and (viii)ability to generate 3D magnetic current maps of boards, packages, ordies using magnetoresistive sensors.

Although the invention has been described with reference to certainexemplary embodiments, it is to be understood that the forms of theinvention shown and described are to be treated as exemplary embodimentsonly. Accordingly, various changes, substitutions and modifications canbe realized without departing from the spirit and scope of the inventionas defined by the appended claims.

1. A method, comprising: applying a stationary stimulus with acontrollable spot size to a device under test (DUT), said controllablespot size covering a portion of said DUT for excitation by saidstationary stimulus, wherein said controllable spot size is regulated byvarying focal control associated with a microscope head to which astimulus source is coupled; and capturing at least one of a functionalresponse signal and an optical image signal emanating from said portionof said DUT, wherein said functional response signal and said opticalimage signal are operative for diagnosing said DUT.
 2. The method asrecited in claim 1, further comprising scanning remaining portions ofsaid DUT until a predetermined area thereof has been traversed.
 3. Themethod as recited in claim 2, wherein said operation of scanningremaining portions of said DUT is performed by moving on an X-Y plane atleast one of said DUT's stage and a microscope head to which a stimulussource is coupled.
 4. The method as recited in claim 3, wherein saidmoving is effectuated by using a servo motor.
 5. The method as recitedin claim 3, wherein said moving is effectuated by using a stepper motor.6. The method as recited in claim 3, wherein said moving is effectuatedby using a linear positioning device.
 7. The method as recited in claim1, wherein said controllable spot size is controlled by moving saidDUT's stage in a vertical direction.
 8. The method as recited in claim1, wherein said stationary stimulus comprises a laser beam.
 9. Themethod as recited in claim 1, wherein said stationary stimulus comprisesan electron beam.
 10. The method as recited in claim 1, wherein saidstationary stimulus comprises a high frequency magnetic induction field.11. The method as recited in claim 1, wherein said stationary stimuluscomprises a thermal field.
 12. The method as recited in claim 1, whereinsaid stationary stimulus comprises an electrostatic field.
 13. Themethod as recited in claim 1, wherein said stationary stimulus comprisesa radio frequency (RF) field.
 14. The method as recited in claim 1,wherein said stationary stimulus comprises a magnetic field.